Gas lift is one of several artificial lift systems that can be employed to produce an oil well which does not have sufficient natural bottom hole pressure to cause the oil to flow up through a production string of tubing to the surface. Gas lift also can be used to increase the production from a well having marginal flow of oil at the surface. In either case, gas under pressure is pumped into the casing-to-tubing annulus at the surface and enters the production tubing downhole via one or more flow regulator-type devices called gas lift valves. The gas is entrained into and mixes with the oil to reduce its density so that available bottom hole pressure is sufficient to cause the oil to flow to the surface at a desired rate. A surface separation facility can be used to separate the lift gas from the oil so that the gas can be reinjected into the annulus. This type of artificial lift is in widespread use in oil producing areas where natural gas is readily available.
The use of gas lift valves as a means of artificial lift is believed to have began in the early 1930's. One of the first types to be used was a differential pressure responsive device having a spring-loaded piston subject on one end to tubing pressure and as its other end to casing pressure. Injection gas enters the valve body through one or more flow restrictions which are smaller than the valve port so that a reduced pressure acts on the valve stem. The valve element normally is held open by the spring and requires a certain differential between annulus gas pressure and the reduced pressure to close. Thus as long as the tubing pressure has been sufficiently reduced by entrained gas that the oil is flowing, the valve remains closed. However when tubing pressure rises to a level where flow might cease, the differential pressure is reduced and the spring opens the valve to admit gas into the tubing and reduce the density of the oil. For a general discussion of the principles and theory of this type gas lift valve, see "Principles of Oil Well Production", Second Ed., Nind, McGraw-Hill, at pp. 192-196, and the disclosure of U.S. Pat. No. 3,559,672 relating to FIG. 9 thereof.
Although differential pressure-type gas lift valves in theory offer the most efficient method of gas lifting known, they have embodied several practical problems which have limited their potential. For example the valve element tends to cycle between open and closed positions before assuming either position, which causes unnecessary wear on the valve parts and erratic operation. Moreover, reopening pressure has been essentially the same as closing pressure which is undesirable during the unloading or start-up phase of a gas lift installation because it can result in multiple valves being open at the same time, which can result in stopping the unloading process without ever reaching the desired point of injection.
In the early 1940's U.S. Pat. No. 2,305,250 addressed some but not all of these problems by proposing the use of unequal areas at the upper and lower ends of the valve piston. This feature caused more positive opening and closing of the valve dement to reduce undesirable cycling. However by the late 1940's the dome pressure operated bellows valve became quite popular and has substantially diminished the market for other types of valves since that time. Although a dome pressure valve is not particularly efficient, efficiency was not an important or controlling criteria when natural gas was cheap and oil production was regulated to only a few days per month. Of course it is quite clear that the economy now has changed radically, and that production of remaining oil must be optimized if adequate profits are to be realized.
An object of the present invention is to provide a new and improved differential pressure gas lift valve that optimizes gas lift operations.
Another object of the present invention is to provide a new and improved differential pressure gas lift valve that includes means to allow the reopening pressure differential to be adjustable while the closing pressure differential remains the same.
Another object of the present invention is to provide a new and improved gas lift valve of the differential pressure type which has a large valve seat size and long stem travel which allow high gas injection rates with minimum pressure drop.
Yet another object of the present invention is to provide a new and improved gas lift valve of the differential type that can be used in high volume oil wells requiring large gas injection rates.